As an antenna mainly used in a frequency band of a microwave or higher, an array antenna where many radiation elements are arranged in a row or in a matrix manner and they are connected by a microstrip transmission line and an array antenna where microwave radiation slots are formed on a waveguide wall at constant intervals have been known. Such an array antenna is constituted so as to achieve improvement in gain by radiating electric waves from a plurality of radiation elements or slots. However, in a flat wave composed of spherical waves radiated from respective radiation elements or slots, there is undulation of a phase on a flat plane perpendicular to a radiation directivity axis and the ripples adversely affects a gain, which results in a tendency that the gain does not increase proportionally to the number of radiation elements or slots.
In view of the above circumstances, a technical problem to be solved occurs in order to reduce a ripple of radiation waves of the array antenna to improve the gain, and an object of the present invention is to solve the above problem.